The reason the VA recognized ALS as a service-connected disability was because this study (“Prospective Study of Military Service and Mortality from ALS” Weisskopf, Ph.D.; et. al) found that veterans over the past century have ALS at almost double the rate as the rest of the population. This clearly points to a service connection. It may be related to vaccinations though at this point they don't know. I agree with you that it ought to include all motor neuron disease.
I emailed the authors of this study and they informed me that they included people with PLS in their research. So people with PLS were included to make the case for service connection but are being excluded from the benefits. Doesn't make much sense!
The list of links I posted a while back indicates that there is a wide body of medical opinion that says that PLS is a form of ALS. These are respected medical organizations such as Johns Hopkins, Merk medical manual...etc not some fly-by-night outfits. I think we need to send this information to our doctors and our congressional representatives. There are not very many of us but if we can get our friends and families to make enough noise I'm sure we can get this changed. Politically both parties are eager to do things for veterans and because we are few in number it is not going to cost them a fortune.